Regenerator is a Pokémon ability introduced in the fifth generation. It recovers the health of the Pokémon when it is switched out.
When a Pokémon with Regenerator is switched out, 1/3 of their health is recovered. When a Pokémon gains the ability through Trace and similar, it will regain the health upon switching out if it isn't suppressed beforehand. Heal Block does not prevent healing from this ability.
In the Pokémon Mystery Dungeon series, the ability heals 20% of the Pokémon's max HP when changing floors.